In the prior art, it has long been recognized that the proper generation and maintenance of the lubricant film between the rolling elements and races of an operating rolling element bearing assembly and between meshing gear teeth are critical with respect to bearing and gear life and maintainability. As a consequence, there have been many attempts in the prior art to measure elastohydrodynamic lubricant film thickness in actual bearings or in apparatus in which the bearings or gears are simulated. Some of these measurement schemes have involved the electrical measurement of resistance, of dielectric breakdown voltage, or of the effective capacitance of the lubricant film under operating conditions. In addition, x-ray diffraction techniques and optical interferometric methods have been applied with certain degrees of success.
Because of their nature, most of the prior art test arrangements have involved specially prepared and instrumented bearing assemblies which usually do not allow detailed study of a lubricant's behavior under conditions typically found in gear and rolling element bearing contacts. One such example of a test device involving bearing simulation is taught in U.S. Pat. No. 3,178,928, issued on Apr. 20, 1965 to T. B. Howe, and assigned to Sperry Rand Corporation. In the Howe apparatus, important characteristics of ball bearings or elements thereof may be investigated by a simulation method so that valuable data may be obtained through measurements of torque and electrical resistance, the measurements providing an indication of operation efficiency. Since the apparatus is a simulation apparatus, interchangeability of simulated bearing components is permitted to some extent. The device provides output data respective to the torque transmitted through the ball elements as well as the electrical resistance to currents passing through the lubricating film associated with those ball elements. Changes in efficiency of the simulated bearing are generally indicated by rising or erratic torque or electrical resistance indications that can be used to predict failure well in advance of the failure event.
However, like many of the prior art test concepts, studies using the Howe apparatus are performed on simulated bearing arrangements and the concept does not readily permit the direct testing of commercially produced bearings or the testing of lubricants directly in such commercial bearings.
The reliability and life of a ball bearing is a function of its lubricating film thickness with respect to minute irregularities in the surface finish of the bearing balls and races. The film thickness of consequence is that of the plateau at the central contact locus of the lubricating film. The trailing edge of this separating film is reduced in thickness by the hydraulic, elastic, hydrodynamic, and other forces involved. It has been determined experimentally that bearing life is most closely associated with the lubrication film thickness in the central plateau region and is not significantly related to that at the exit region restriction which is normally measured by electrical resistance measurements. Thus, electrical resistance measurements, while useful for many purposes, are often not as accurate an indicator of bearing life as is desired. Furthermore, it is not possible to search a bearing race for local imperfections, such as for areas in which a sufficient lubrication film does not exist due to an oleophobic surface, misalignments, or local raceway surface defects.
It is recognized that ultrasonic energy has been employed in the past for various testing purposes, such as, for example, the detection of hidden flaws deep within solid structures. In such conventional ultrasound test equipment, for example, a pulse of energy is directed through a medium toward a defect or other interface and the arrival time of the reflected wave at a receiver is measured in order to afford a measurement of the depth of the flaw or other interface. However, the technique lends itself to use only in situations in which the flaw or interface to be observed is buried relatively deeply within the medium. Thus, the direct measurement of films as thin as bearing lubrication films would not prove to be practical using this conventional technique.
Jacobson in U.S. Pat. No. 3,952,566 discloses a method and apparatus where a bearing and elastohydrodynamic lubrication film are measured by relating acoustic resonance frequencies of a ball raceway lubricant system of a ball bearing, which frequencies are found to be related to bearing ball, race, and lubricant behavior. These measurements are made while the bearing is actually operating in its normal environment such as for example in a production gyroscopic instrument assembly.
Previous configurations as mentioned above consist of either balls or rollers loaded against a flat face of a disc of alternatively a number of discs forming an external contact on their circumferences. Use of ball or roller on disc configurations limit the speed at which measurements can be made due to centrifugal forces experienced by the lubricant. At high speeds lubricant is forced away from metal contact which contact becomes starved of lubricant. External contact of discs allows high speed traction measurements but it is difficult to view the contact directly using optics.
Therefore, what is needed is a method and apparatus which eliminates lubricant starvation due to centrifugal forces while allowing direct measurement of traction and optical film thickness measurements to be made under EHL conditions.